The Proline-rich acidic protein (PRAP) gene was first identified in the mouse as a pregnant-specific uterine protein (psup) (Kasik and Rice, 1997). The gene was expressed in the mouse uterus from day 12 of pregnancy to the third day after parturition. Studies conducted in the laboratory demonstrated that PRAP expression was not limited to the late pregnant uterus (Zhang et al., 2000). PRAP mRNA was found abundantly expressed in the proximal small intestine of both the rat and mouse. Expression was highest in the proximal small intestine. Expression decreased distally and was undetectable in the terminal ileum in Northern blots.
The PRAP gene encodes a novel protein expressed at high concentrations in human kidney, liver and the gastrointestinal tract. It has no homology to known proteins. However, the presence of a signal peptide in the N-terminal part of the protein suggests that it is a secreted protein.
Epigenetic mechanisms are integral in to the process of tumour development (Jones and Bayin, 2002). Epigenetic modifications do not change the DNA sequence itself but alter the transcriptional activity of genes, changing the repertoire of genes expressed by the cell. The two major epigenetic mechanisms operating in the cell are DNA methylation at CpG islands in the promoter and histone acetylation. Methylation at CpG islands silences gene transcription in most instances (Hark et al., 2000), but, rarely, it results in activation (Hark et al., 2000; Plass et al., 1996). Similarly, deacetylation of histones is thought to result in transcriptional silencing because of the condensation of chromatin (Gray and Ekstrom, 2001; Cress and Seto, 2000; Wolffe and Pruss, 1996; Grunstein, 1997; Brownell and Allis, 1996). Recent evidence suggests that the two processes are related. Methylated DNA appears to preferentially associate with histone deacetylase protein complexes and histone methylases (Nan et al., 1998; Jones et al., 1998; Robertson et al., 2000). Histone methylation has been shown to result in DNA methylation (Tamaru et al., 2001; Jenuwein, 2001).
Epigenetic mechanisms play important roles in the regulation of genes involved in the development of cancer. Its importance is underscored by the reversal of the cancer phenotype when the cells are treated with drugs that modify epigenetic mechanisms (Marks et al., 2001; Johnstone, 2002; Marks et al., 2000). Genes that are epigenetically silenced during carcinogenesis are potential tumour suppressors. The identification of these genes facilitates the design of therapeutic strategies to combat cancer. There is therefore renewed interest to identify genes that are silenced in cancer, especially those involved in differentiation and cell proliferation (Yamashita et al., 2002).
U.S. Pat. No. 5,856,139 and US patent application 2002/0115153 describe a human PRAP gene and the encoded protein. These documents disclose the use of PRAP or PRAP agonists to stimulate cell proliferation. They also disclose the use of PRAP antagonists to treat cancer or prevent cell proliferation.
The role of PRAP in regulating cell proliferation has been very unclear to recommend specific treatment strategies using this molecule and therefore there is a demand for investigation which would clarify that role.